Optical signal transmission of data can have advantages over electrical transmission of data, particularly when data must be transmitted at very high rates. These advantages are usually associated with the wide bandwidth required in the transmission system, wherein transmission lines used for electrical data transmission are more subject to noise and signal attenuation than the optical fibers used for optical data transmission.
Light sources for such optical data systems can be directly or indirectly modulated by electrical signals that represent the transmitted data. Indirect modulation involves the use of an optical modulator that responds to the electrical signals that represent the data to be transmitted. Such optical modulators are typically of the electro-absorptive, electro-dispersive, or phase-shift type. Regardless of type, the optical modulator is usually configured to cause a variation or shift in the intensity of the optical beam from the light source in some relationship to the electrical signals.
Compact optical data transmitters of the indirectly modulated type preferably comprise a laser diode light source that is coupled to an electro-absorptive waveguide type modulator. Typically, the electro-absorptive modulator operates by utilizing the shift in transmissivity of the modulator's waveguide to a longer wavelength due to the application of a strong electric field.
This shift occurs with modulator waveguides of several different semiconductor structures, such as when the structure of the waveguide comprises a bulk semiconductor, a single isolated quantum well, multiple isolated quantum wells, or multiple coupled quantum wells (a superlattice). With this type of modulator, the wavelength at which the modulator's waveguide changes from relatively transmissive to relatively opaque changes as a function of the potential of the electrical input signals applied to it.
Thus, for any given instantaneous potential applied to the electrical input of the modulator, there is a range of wavelengths of light that may be passed through the modulator's waveguide with relatively low absorption, a range of wavelengths with relatively high absorption, and a narrow range of wavelengths at which the characteristics of the waveguide shift from relatively transmissive to relatively absorptive, an "electro-absorptive edge" region.
If an operating wavelength for the light source is selected so that the change in potential of the electrical input signals, plus any applied bias potential, shifts or varies the electro-absorptive edge about this operating wavelength, a modulated optical signal that has a relatively large depth of modulation is generated by the modulator. The modulator is biased to shift the wavelengths of the electro-absorptive edge with respect to the operating wavelength so that a relatively small change in input signal causes a relatively large change in absorption of the modulator.
Such optical data transmitters often require substantial optical powers to be coupled into the modulator. For example, to provide a modulated output signal of between 100 microwatts and 1 milliwatt coupled into an optical fiber from the modulator may require approximately 10 to 20 milliwatts to be coupled into the modulator from the light source. Practical combinations of electrical input signal levels and optical input power can easily damage the modulator under ordinary operating conditions.
It has been determined that the damage that occurs to the modulator occurs primarily at or near the input facet of the modulator. This modulator damage may be reflected in its optical properties, its electrical properties, or both. The damage is believed to be due to excessive photo-current that is due to both the optical input power and the electrical signal input potential. Therefore, it is desirable to modify the construction of the modulator to minimize such damage to the modulator proximate its input facet under operating conditions.